Essential Cell Biology 5th edition

Questions
265
QUESTION 7–16
A. The average molecular weight of a protein in the cell is
about 30,000 daltons. A few proteins, however, are much
larger. The largest known polypeptide chain made by any
cell is a protein called titin (made by mammalian muscle
cells), and it has a molecular weight of 3,000,000 daltons.
Estimate how long it will take a muscle cell to translate
an mRNA coding for titin (assume the average molecular
weight of an amino acid to be 120, and a translation rate of
two amino acids per second for eukaryotic cells).
B. Protein synthesis is very accurate: for every 10,000
amino acids joined together, only one mistake is made.
What is the fraction of average-sized protein molecules and
of titin molecules that are synthesized without any errors?
[Hint: the probability P of obtaining an error-free protein is
given by P = (1 – E) n , where E is the error frequency and n
the number of amino acids.]
C. The combined molecular weight of the eukaryotic
ribosomal proteins is about 2.5 × 10 6 daltons. Would it be
advantageous to synthesize them as a single protein?
D. Transcription occurs at a rate of about 30 nucleotides
per second. Is it possible to calculate the time required to
synthesize a titin mRNA from the information given here?
QUESTION 7–17
Which of the following types of mutations would be
predicted to harm an organism? Explain your answers.
A. Insertion of a single nucleotide near the end of the
coding sequence.
B. Removal of a single nucleotide near the beginning of the
coding sequence.
C. Deletion of three consecutive nucleotides in the middle
of the coding sequence.
D. Deletion of four consecutive nucleotides in the middle of
the coding sequence.
E. Substitution of one nucleotide for another in the middle
of the coding sequence.
QUESTION 7−18
Figure 7−8 shows many molecules of RNA polymerase
simultaneously transcribing two adjacent genes on a single
DNA molecule. Looking at this figure, label the 5ʹ and 3ʹ
ends of the DNA template strand and the sets of RNA
molecules being transcribed.